1. Field of the Invention
The present invention relates to a global imaging apparatus and, more particularly, to a microscopic imaging apparatus having a optical beam with flat-top intensity distribution, which is suitable for applications of various fields, such as fluorescence or Raman system of global imaging.
2. Description of Related Art
The chemical compositions, impurities, and defects existing in target materials cannot be identified and inspected by traditional optical methods, and must be measured by chemical imaging techniques. By using the laser excited electronic state spectrum or the vibration state spectrum, the chemical bonding information of sample is obtained. Among three measured methods (i.e. the point scan, line scan and global imaging) in recent chemical imaging technologies, the global imaging method has the highest image acquisition speed under the same photo-energy density (W/cm2). It is because the chemical image of larger illuminated area can be caught directly by array detector. The sample scanning or incident beam scanning is not needed for the global imaging. Thus, the global image has a superior measured speed property.
However, in most cases, the incident light is a Gaussian beam, whose cross section has a stronger intensity distribution at the center of a light beam than that at the edge, as shown in FIG. 1. As a result, the chemical image excited by Gaussian beam produces severe non-uniformality, which often causes a misreading of the concentration of sample. As shown in FIG. 2, the spectrum of sample A or C at the edge is different from the spectrum of sample B in the center. This property limits the development of the global imaging method. Thus, it is necessary to find a new method for solving the non-uniformity problem.
For prior chemical image measurement systems, there are three methods to overcome the problems caused by non-uniform beams. First, by using a Powell lens, the central part of the laser beam can diverge faster than the edge part does. A laser beam with an approximate flat-top intensity distribution is obtained according to the Snell's law, i.e. laws of optical refraction at surface. However, the manufacture of a Powell lens is difficult, and only a two-dimensional Powell lens can be obtained. The laser beam can only be modified into a uniform line source. This method can only be applied in the scope of line scan chemical image, and cannot be put into application at the global imaging.
Another improvement can be achieved by using a natural density filter with a specialized distribution for optical attenuation. The light attenuation of filter at the center is stronger than that at the edge, the intensity of laser beam is transformed into approximate flat-top distribution. The method is applied to the chemical imaging spectrum system of the global imaging. But the components having a low damage threshold and high absorption from light heating can only be applied to the fluorescent chemical imaging system which has low laser illumination power. It cannot be used in the Raman chemical imaging system having high laser illumination power.
The third way for improving the intensity distribution of the beam is to project a Gaussian beam into a holographic optical element, on which there is an interference spectrum record of the Gaussian beam and the flat-top beam for obtaining a recovered flat-top beam. However, both the energy conversion efficiency and the damage threshold of the components are low. It also cannot be effectively used to all of the global image systems.
Therefore, it is desirable to provide a microscopic imaging apparatus to mitigate and/or obviate the aforementioned problems.